Ultrathin TLA Materials Could Broaden IR Applications

Ultrathin gratings composed of common materials were shown to increase the absorption efficiency of light to almost 99 percent when thin grooves were etched into the film, directing the light sideways. The semiconductor materials are compatible with optoelectronic applications such as photodetectors and optical modulators, and could make IR technology less expensive and more accessible.

A team comprising researchers from the University of Sydney, Australia National University and University of Technology Sydney began their investigation by examining total light absorption (TLA) in homogeneous ultrathin films, finding that TLA was difficult to achieve in uniform ultrathin layers.

When light falls on a very thin, uniform layer almost all of it is reflected (right-hand arrows). By etching thin grooves in the film, the light is directed sideways and almost all of it is absorbed (left-hand arrow) even though the amount of material is very small. Insets show electron micrographs of the structuring. The absorbing layer is only 0.041-µm thick. Courtesy of Thomas P. White, Australian National University.

The team then turned their attention to producing ultrathin gratings of common materials. They demonstrated TLA of TE (transverse electromagnetic) polarized light using antimony sulphide (Sb₂S₃) semiconductor gratings, placed above a metal reflector. They reported that the asymmetric configuration allowed for TLA with only a single incident beam.

The researchers then fabricated a 41-nm-thick antimony sulphide grating structure with a measured absorptance of A = 99.3 percent at the visible wavelength of 591 nm. The results showed that the absorption within the grating was A = 98.7 percent, with only A = 0 .6 percent within the silver mirror. In contrast, a planar reference sample absorbed only A = 7.7 percent at this wavelength.

The research team also investigated the range of material parameters that might be compatible with TLA in ultrathin gratings. Results showed that ultrathin gratings made of a wide range of weakly absorbing semiconductors could absorb nearly 100 percent of TE-polarized light.

"Conventional absorbers add bulk and cost to the IR detector as well as the need for continuous power to keep the temperature down,” said professor Martijn de Sterke. “The ultrathin absorbers can reduce these drawbacks."

Researcher Björn Sturmberg said the findings did not rely on a particular material but could be applied to many naturally occurring weak absorbers.

"There are many applications that could greatly benefit from perfectly absorbing ultrathin films, ranging from defense and autonomous farming robots to medical tools and consumer electronics," Sturmberg said.

The near-perfect absorption of light in subwavelength thickness layers generally relies on exotic materials, metamaterials or thick metallic gratings. The structures developed by the Australian team are simpler to design and fabricate than existing thin-film light absorbers, which may require complex nanostructures or difficult-to-create combinations of metals and nonmetals, in addition to exotic or metamaterials. High-quality IR detectors cost approximately $100,000 and some require cooling to −200 °C. Significant cost, efficiency and sensitivity gains may be possible from making photodetectors using less material.

The research was published in Optica, a journal of the Optical Society of America (OSA) (doi: 10.1364/optica.3.000556)